How to know when a component has been made invisible or visible ???

In this example, we use button as component.

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COMPLETE SOURCE CODE FOR : HowToKnowWhenComponentVisible.java
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import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JOptionPane;

import java.awt.event.ComponentListener;
import java.awt.event.ComponentAdapter;
import java.awt.event.ComponentEvent;

import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;

import java.awt.FlowLayout;

public class HowToKnowWhenComponentVisible
{
 JFrame a=new JFrame("CLICK HERE");
 JFrame b=new JFrame("FRAME TO STORE JBUTTON NAME'S MONSTER");
 
 JButton c=new JButton("VISIBLE");
 JButton e=new JButton("INVISIBLE");
 JButton d=new JButton("MONSTER");
 
 public HowToKnowWhenComponentVisible()
 {
  ComponentListener cl=new ComponentAdapter()
  {
   public void componentHidden(ComponentEvent evt)
   {
    JOptionPane.showMessageDialog(null,"MONSTER IS INVISIBLE");
   }
   
   public void componentShown(ComponentEvent evt)
   {
    JOptionPane.showMessageDialog(null,"MONSTER IS VISIBLE");
   }
  };
  
  d.addComponentListener(cl);
  b.add(d);
  
  ActionListener al=new ActionListener()
  {
   public void actionPerformed(ActionEvent evt)
   {
    if(evt.getSource()==e)
    {
     d.show(false);
     b.repaint();
    }
    
    if(evt.getSource()==c)
    {
     d.show(true);
     b.repaint();
    }
   }
  };
  
  c.addActionListener(al);
  e.addActionListener(al);
  a.setLayout(new FlowLayout());
  a.add(c);
  a.add(e);
  
  a.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  a.setSize(400,100);
  a.setVisible(true);
  
  b.setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
  b.setSize(400,100);
  b.setLocation(400,400);
  b.setVisible(true);
 }
 
 public static void main(String[]args)
 {
  HowToKnowWhenComponentVisible myObject=new HowToKnowWhenComponentVisible();
 }
}

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JUST COMPILE AND EXECUTE IT
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